Datasheet

 1999-2013 Microchip Technology Inc. DS39026D-page 151
PIC18CXX2
15.1 USART Baud Rate Generator
(BRG)
The BRG supports both the Asynchronous and Syn-
chronous modes of the USART. It is a dedicated 8-bit
baud rate generator. The SPBRG register controls the
period of a free running 8-bit timer. In Asynchronous
mode, bit BRGH (TXSTA<2>) also controls the baud
rate. In Synchronous mode, bit BRGH is ignored.
Table 15-1 shows the formula for computation of the
baud rate for different USART modes, which only apply
in Master mode (internal clock).
Given the desired baud rate and F
OSC, the nearest
integer value for the SPBRG register can be calculated
using the formula in Table 15-1. From this, the error in
baud rate can be determined.
Example 15-1 shows the calculation of the baud rate
error for the following conditions:
•F
OSC = 16 MHz
• Desired Baud Rate = 9600
• BRGH = 0
• SYNC = 0
It may be advantageous to use the high baud rate
(BRGH = 1), even for slower baud clocks. This is
because the F
OSC/(16(X + 1)) equation can reduce the
baud rate error in some cases.
Writing a new value to the SPBRG register causes the
BRG timer to be reset (or cleared). This ensures the
BRG does not wait for a timer overflow before output-
ting the new baud rate.
15.1.1 SAMPLING
The data on the RC7/RX/DT pin is sampled three times
by a majority detect circuit to determine if a high or a
low level is present at the RX pin.
EXAMPLE 15-1: CALCULATING BAUD RATE ERROR
TABLE 15-1: BAUD RATE FORMULA
TABLE 15-2: REGISTERS ASSOCIATED WITH BAUD RATE GENERATOR
Desired Baud Rate = FOSC / (64 (X + 1))
Solving for X:
X = ( (F
OSC / Desired Baud rate) / 64 ) - 1
X = ((16000000 / 9600) / 64) - 1
X = [25.042] = 25
Calculated Baud Rate = 16000000 / (64 (25 + 1))
= 9615
Error = (Calculated Baud Rate - Desired Baud Rate)
Desired Baud Rate
= (9615 - 9600) / 9600
=0.16%
SYNC BRGH = 0 (Low Speed) BRGH = 1 (High Speed)
0
1
(Asynchronous) Baud Rate = F
OSC/(64(X+1))
(Synchronous) Baud Rate = F
OSC/(4(X+1))
Baud Rate = F
OSC/(16(X+1))
NA
Legend: X = value in SPBRG (0 to 255)
Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value on
POR,
BOR
Value on all
other
RESETS
TXSTA
CSRC TX9 TXEN SYNC —BRGHTRMT TX9D 0000 -010 0000 -010
RCSTA SPEN RX9 SREN CREN ADDEN FERR OERR RX9D 0000 -00x 0000 -00x
SPBRG Baud Rate Generator Register 0000 0000 0000 0000
Legend: x = unknown, - = unimplemented, read as '0'. Shaded cells are not used by the BRG.